Remove FormComponent persistence

Details

Description

At the very beginning of Wicket we introduced FormComponent.setPersistence with the idea to make creating Login panel very easy. Basically Wicket would deal with the Cookie handling for the login details. Now some years later I think it is evident that this feature is not very useful:

it basically is used for login panels only

even in our examples Password.setPersistence is false which means that "rememberMe" doesn't really work. Only the user name is stored in a Cookie

forcing formComponent.getPageRelativePath() as the Cookie name is quite restrictive

you can not combine username and password into a single Cookie and e.g. decrypt it for safety reasons

I'd like to propose to remove all that code from core.

I've attached a patch for 1.4RC2 which remove that code and I updated SignInPanel and the example to show how easy it still is.